In an institute of electrical and electronics engineers (IEEE) 802.16 task group that chooses the next-generation wireless interface standards and a nonprofit worldwide interoperability for microwave access (WiMAX) forum that provides service and network standards for a broadband wireless connection system based on IEEE 802.16, standardization has been being achieved for a wireless connection system supporting a femto-cell to enhance efficiency of the system and improve quality of service (QoS) under an indoor environment. The WiMAX forum defines a femto base station (BS) as a low-power inexpensive base station connected to an Internet protocol (IP) network through a fixed wireless link or a local broadband wired link.
FIG. 1 shows a network structure including a femto-cell system. The femto BS is connected to the IP network spread in a home or an office, and accesses a core network of a mobile communication system through the IP network, thereby providing mobile communication service. That is, the femto BS may be connected to the core network of the mobile communication system through a digital subscriber line (DSL). A user of the mobile communication system may receive the service in the outdoors through the existing macro-cell, but receive the service in the indoors through the femto-cell. The femto-cell improves indoor coverage of the mobile communication system by making up for that the service of the existing macro-cell becomes deteriorated in a building, and provides the service intended for the a specific user, thereby providing voice and data service of high quality. Further, the femto-cell can provide a new service not given in the macro-cell, and the spread of the femto-cell may cause fixed-mobile convergence (FMC) to be accelerated and an industrial-based cost to be reduced.
In a mobile wireless access system supporting the femto-cell, the femto BS has a power off state, an initialization state and an operational state. The power off state refers to a state before the femto BS is powered on or a state that the femto BS is powered off from the initialization state or the operational state. That is, the power off state refers to that no more service is given to user equipment (UE). The initialization state refers to an entry state once the femto BS starts receiving power, in which an initial system parameter acquisition, physical(PHY)/medium access control (MAC) layer parameter decision, topology acquisition or the like wireless interface setting is performed, and time and frequency synchronization using a global positioning system (GPS), a wired interface, IEEE 1588 or the like is performed.
If the femto BS successfully accesses a network, the femto BS enters the operational state. In the case that one or more UEs access coverage of the femto BS and receive service, the femto BS enters a normal operation mode and serves as a BS. Also, the femto BS supports a low duty operation (LDO) mode to decrease interference with neighbor cells. The femto BS, which enters the LDO mode, alternates between an availability interval (AI) and an unavailability interval (UAI). The femto BS entering the AI operates in an active mode. On the other hand, the femto BS entering the UAI does not transmit and receive any information with the UE through an air interface to minimize the interference.
The femto BS of the normal operation mode can enter the LDO mode when all the UE that belong to the femto BS are in a sleep mode or an idle mode or when no UE belongs to the femto BS. The femto BSs, which enter the LDO mode, may be different in the AI and the UAI from one another in accordance with conditions of the BS. That is, the femto BSs in the LDO mode may have different patterns of the AI and the UAI according to the BSs.
The above related arts refer to an entry condition of the femto BS from the operational state into the normal operation mode and the LDO mode and an operation method for the LDO mode. However, since the above related art is nothing but mention about general technology, there are further needed detailed descriptions about a switching condition of the operation state to be considered in the practical femto BS, a determining method for an LDO cycle as a basis of the operation of the LDO mode, etc.